Transition State Modeling for Catalysis - American Chemical Society

Chapter 11

Theoretical Studies of Inorganic and Organometallic Reaction Mechanisms 13: Methane, Ethylene, and Acetylene Activation at a Cationic Iridium Center 1

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Shuqiang Niu, Douglas L. Strout , Snežana Zarič , Craig A . Bayse, and Michael B. Hall

Downloaded by COLUMBIA UNIV on August 7, 2012 | http://pubs.acs.org Publication Date: April 8, 1999 | doi: 10.1021/bk-1999-0721.ch011

Department of Chemistry, Texas A&M University, College Station, TX 77843-3255

The oxidative-addition/reductive-elimination (OA/RE) reactions of methane, ethylene and acetylene with the CpIr(PH )(CH ) complex are investigated by ab initio methods and density functional theory (DFT). The calculated results shows that the OA reaction from CpIr(PH )(CH )(agostic-alkane) to CpIr(PH )(CH )(H)(alkyl) is endothermic by 4.4 and 0.8 kcal/mol with a low barrier of 11.5 and 10.0 kcal/mol at the DFT-B3LYP and coupled cluster with singles and doubles (CCSD) levels of theory, respectively. The RE reaction from CpIr(PH )(CH )(H)(alkyl) to a β-agostic complex, CpIr(PH )(alkyl) , is exothermic with a low barrier of 7.1 and 9.2 kcal/mol. A strong stabilizing interaction between either ethylene or acetylene and CpIr(PH )(CH ) leads to a high activation barrier (24-36 kcal/mol) for the OA processes of either one. Compared to ethylene, the OA/RE reaction of acetylene with CpIr(PH )(CH ) complex is more favorable. Thus, the dimerization of terminal alkynes catalyzed by cationic iridium complexes is plausible. +

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Transition metal-catalyzed reactions can be regarded as consisting of several elementary reactions such as oxidative addition, reductive elimination, migratory insertion, β-hydride elimination, σ-bond metathesis, and nucleophilic addition (1). Numerous experimental and theoretical studies have been undertaken in order to understand these pivotal fundamental steps (1,2). Because of enormous progress in computational chemistry, today one can also determine accurate geometries and relative energies of the species involved in these reactions from first principles. This is especially important in those cases where experimental results are difficult to obtain (1,3). 1

Current address: Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, WA 99352. Permanent address: Faculty of Chemistry, University of Belgrade, Studentski trg 16,11001 Beograd, Yugoslavia. 2

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© 1999 American Chemical Society

In Transition State Modeling for Catalysis; Truhlar, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

139 Both early and late transition metals undergo many of the elementary reactions mentioned above, but the mechanism often differs in important details. In some instances, the differences are so large that the reactions are only superficially similar. For example, consider carbon—hydrogen bond activation, both early and late transition metals can break C — H bonds, but the mechanism is very different. Early transition metals, often d° system such as Sc(III), activate C — H bond through a a-bond metathesis (4) as shown in equation 1. •H

[M]—CH

3

+

+ RH


[M]---CH

CH

4

(1)

3

By contrast, late transition metals, such as Ir(I) or Rh(I), accomplish C — H bond activation through an oxidative-addition (5) process as shown in equation 2. Thus, the early metals more often display catalytic hydrogen exchange while the late metals, especially the heavier ones, tend toward stoichiometric reactions. H

[M]—CH

3

+ RH-

I

R — [ M ]—CH

[M]—R

+ CH

4

(2)

:

Bergman and co-workers recently reported an Ir(III) system which catalyzes both hydrogen exchange in methane and alkanes in the solution-phase at room temperature and generates olefin complexes (6). It shows a striking similarity in hydrogen exchange to the reaction of early metals. Because the behavior of this Ir(III) system, C p * I r ( P R ) ( C H ) [Cp* = -n -C (CH3)5], is quite different from the now more common Ir(I) systems, Bergman and co-workers suggested two possible reaction mechanisms: (i) the reaction resembles an early metal reaction and proceeds through a abond metathesis, as in equation 1, or (ii) the reaction resembles a late metal and proceeds through an oxidative-addition/reductive-elimination (OA/RE) pathway, as in equation 2. The former is unexpected for a late metal, while the latter involves Ir(III) going to Ir(V), a fairly high oxidation state for an Ir system without electronegative ligands. Recently, several experimental and theoretical studies have been performed to elucidate the details of this reaction mechanism (7). In this work, the hydrogen exchange of methane (1), ethylene (2), and acetylene (3) with the Ir(III)-methyl complex are investigated with density functional theory (DFT) and ab initio molecular orbital theory. +

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Computational Details The geometry optimizations in this work have been performed using DFT (8) methods, specifically the Becke three parameter hybrid exchange functional (8b-d) and the Lee-Yang-Parr correlation functional (B3LYP) (8e). The transition states were optimized using a quasi-Newton method and characterized by determining the number of imaginary frequencies (9). For a direct estimation of electron correlation effects and accurate reaction energetics, coupled cluster with singles and doubles (CCSD) (10) calculations were carried out on the DFT-optimized geometries. To simplify calculations we replaced the phosphine group and the Cp* ring in the actual molecules by a P H 3 group and a Cp ring (Cp = T | - C 5 H 5 ) (11). Differences in the zero-point 5


140 energy (ZPE) and thermal corrections to the reaction energy tend to be small, so they are not included in the energy calculations. For example, we determined ZPE and thermal corrections for the transformations H2Ti(CHCH2)(H) -> H2Ti(T| -CH2CH2) and 2

2

H Ti(Ti -CH2CH )(H)+ -> H T i C H C H . The ZPE corrections were 2.6 and 2.5 kcal/mol and the thermal corrections were 2.0 and 1.4 kcal/mol, respectively (12). Iridium was described by a modified version of the Hay and Wadt basis set with effective core potentials (ECP) (13a). The modifications to the double-^ basis set were made by Couty and Hall (13b) and give a better representation of the 6p space. The result is a [3s3p2d] contracted basis set for iridium, where the 5s and 5p basis functions are left totally contracted but the 6s, 6p, and 5d are split (41), (41), and (21), respectively. The carbons and hydrogens are described using the Dunning-Hay double£ basis functions (14a). The Hay and Wadt E C P double-^ basis set is used for phosphorus (14b). At the CCSD//B3LYP level, the association energies of methane, ethylene, and acetylene with CpIr(PH3)(CH3) complex have been corrected for the basis-set superposition error (BSSE) (14c). A l l ab initio and DFT calculations were performed with G A M E S S - U K (15) and GAUSSIAN94 programs (16), at the Cornell Theory Center on I B M ES6000 and Scalable Powerparallel (SP2), at Cray Research, Inc. on the Cray C90, at the Supercomputer Center of Texas A & M University and the Department of Chemistry on Silicon Graphics Power Challenge servers, and on Silicon Graphic Power Indigo II I M P A C T 10000 workstations in our laboratory and at the Institute of Scientific Computation (ISC) of Texas A & M University. 2


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Results and Discussion A. Methane C — H Bond Activation. The experimental work of Bergman and co-workers shows that alkanes can be activated by Cp*Ir(PMe3)(CH3) at room temperature to generate olefin complexes (6). The reaction initially produces a new alkyl Ir complex through either oxidative-addition/reductive-elimination (OA/RE) or G +

bond metathesis and then produces an Ir-olefin complex through p-H transfer as illustrated in Scheme 1. Here, the methane C — H bond activation process by the two mechanisms will be examined. Scheme 1


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The B 3 L Y P optimized geometries for the reactant (4), agostic intermediate (5), transition state (6), and oxidative-addition intermediate (7) along the OA/RE pathway are shown in Figure 1.

Figure 1. The B 3 L Y P optimized geometries for the reactant (4), agostic intermediate (5), transition state (6), and oxidative-addition intermediate (7) along the O A / R E pathway (only the average C — C and C — H distances are given for the Cp ring).

The complete pathway for methyl exchange at the iridium center is symmetric with respect to the oxidative-addition intermediate (7). In intermediate 5, a C — H bond of methane is close to the metal center with an Ir—H distance of 2.007 A, and the inside C — H distance is longer by 0.034 A than that of a free methane molecule. Such structural features point to the presence of an Ir—H—C agostic interaction in 5 (17). In transition state 6, there is significant Ir—C and Ir—H bond making. In intermediate 7, the C — H bond is completely broken, while the Ir—H bond and a new Ir—CH3 bond are fully formed. It is noteworthy that the Cp ring of 7 is clearly slipped with respect to reactant 4 as shown in Scheme 2 (18a). Clearly, with the oxidative-addition going from 1 + 4 to 7, the Ir—Cp and Ir—CH3 distances of 5, 6, and 7 are increasing and the Cp ring slips more at each step. Scheme 2 ~1 +

R = CH , C H 3

2

3t

C H 2

Ir(V)

The energy profile at the B 3 L Y P level is presented in Figure 2 and the relative energies at the B 3 L Y P and CCSD//B3LYP levels are summarized in Table I (Reaction 1). Compared to the reactants (1 and 4), the agostic structure 5 is more stable by 1.0


142 and 6.4 kcal/mol at the B3LYP and CCSD//B3LYP levels. However, after corrections for the basis-set superposition error (BSSE) at the CCSD//B3LYP level, 5 is only 0.9 kcal/mol more stable than 1 and 4. Calculations of the alkane association energy in better basis sets would likely yield a value close to 4 kcal/mol for methane and higher values for larger alkanes (18b). The data for the C — H activation of methane (1) shows that the barrier to bond activation, with respect to the agostic structure, is similar for both B 3 L Y P and CCSD//B3LYP, 11.5 and 10.0 kcal/mol, respectively. The oxidativeaddition intermediate (7) is slightly less stable, by 4.4 (B3LYP) and 0.8 kcal/mol (CCSD//B3LYP), than the agostic one (5). The steric effect between the Cp ring and larger phosphine ligand used in the experimental work will lead to an increase in the energy difference between 7 and 5 (19a), which is consistent with the fact that intermediate 7 has not been observed experimentally (6). On the other hand, a decrease of the steric effect between the phosphine and alkyl ligands may lead to an increase in stability of an Ir(V) complex. According to our results, the CpIr(PH3)(H)2(CH3) complex could be trapped at low temperature (19b). The reductive-elimination from 7 through the OA/RE TS, 6, to the agostic complex, 5, is exothermic with a low barrier of 7.1 and 9.2 kcal/mol for B3LYP and CCSD//B3LYP, respectively.


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Table I. Relative Energies (AE) of the Reactions (l)-(3) by B 3 L Y P and CCSD//B3LYP (kcal/mol). CCSD//B3LYP Structure B3LYP AE AE The C — H Activation Reaction of Methane: Reaction 1 0.00 0.00 CpIr(PH )(CH ) , C H 1+4 -1.03 -6.43 (-0.90) CpIr(PH )(CH3)(CH ) 5 10.51 3.54 CpIr(PH )(CH3)(H)(CH ) 6 (TS5.7) 3.38 -5.62 CpIr(PH )(CH )(H)(CH3) T The C — H Activation Reaction of Ethylene: Reaction 2 0.00 0.00 CpIr(PH )(CH ) , C H 2+4 -32.04 -43.08 (-28.68) CpIr(PH3)(CH3)(C H4) 8 1.84 -8.57 CpIr(PH3)(CH3)(H)(C H3) 9 (TS -io) -1.88 -13.54 CpIr(PH3)(CH3)(H)(C H3) 10 4.89 -4.45 CpIr(PH )(CH3)(H)(C H3) H (TS10-12) -10.14 -12.04 CpIr(PH3)(CH4)(C H3) 12 The C — H Activation Reaction of Acetylene: Reaction 3 0.00 0.00 CpIr(PH )(CH ) , C H 3+4 -27.97 -32.63 (-20.05) CpIr(PH3)(CH3)(C H2) 13 -4.31 -0.01 CpIr(PH )(CH3)(H)(C H)+ 14 ( T S 1 3 - 1 5 ) -9.46 -17.06 CpIr(PH3)(CH3)(H)(C H)+ 15 -3.76 -9.36 CpIr(PH )(CH )(H)(C H) 16 ( T S 1 5 - 1 7 ) -22.35 -18.74 CpIr(PH )(CH )(C H)+ 17 a. The BSSE correction is included. +

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Despite a careful search for the pathway of a a-bond metathesis mechanism along a reaction coordinate (RC) involving both Ir—H and C — H distances, we could only find a monotonic increase in energy. This result which implies that the OA/RE pathway is the only plausible mechanism for the hydrogen transfer (7a, 19a). Thus, with confidence we conclude that the a-bond metathesis pathway does not exist for the hydrogen transfer process by the CpIr(PH3)(CH3) complex. +



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Figure 2. The B 3 L Y P energy profiles along the OA/RE and p-H transfer pathways from reactants, 1 and 4, to 7. B. Ethylene C — H Bond Activation. The possible reactions of an iridiumalkene complex may result in new complexes and products as illustrated in Scheme 3. Scheme 3

} ^ CH HP

HP

3

/;«;-«CH H3P

v

-

ethylene/acetylene

3

3

I

3

* *

>5

Polymerization or Oligomerization

I

-CH

4

I

Ci

- UA ethylene/acetylene

H3P

10

In this part, we will consider and examine the possible steps in the ethylene C—H bond activation process. The B 3 L Y P optimized geometries for the 71-complex (8), oxidative-addition TS (9), oxidative-addition intermediate (10), reductive-elimination TS (11), and reductiveelimination product (12) along the OA/RE pathway are shown in Figure 3. The Irethylenerc-complex,8, displays very similar structural features to complex 7 in the longer Ir—Cp and Ir—CH3 bonds, which are characteristic of an Ir(V) complex as mentioned above (see Scheme 2). The substantial difference between the Tt-complex (8) and that of an early transition metal arises because of the particularly strong backbonding between ethylene and Ir. Compared to a free ethylene molecule, the C—C


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double bond of 8 is longer by 0.080 A, and the distances between Ir and the carbons of the ethylene are close to that of normal Ir—C bonds.

Figure 3. The B 3 L Y P optimized geometries for the ^-intermediate (8), oxidative-addition TS (9), oxidative-addition intermediate (10), reductiveelimination TS (11), and reductive-elimination product (12) along the OA/RE pathway (only the average C—C and C — H distances are given for the Cp ring). As ethylene rotates to put a C — H bond close to the metal center, the C — H bond begins to break and the Ir—H bond begins to form on reaching the transition state (9). Compared to the TS, 6, the TS of ethylene C — H bond activation, 9, is slightly later, where the C — H distance is longer by 0.083 A than that of 6. The oxidative-addition intermediate, 10, is similar structurally to the alkane oxidative-addition intermediate, 7. As the methyl ligand migrates and the hydrogen inserts into the Ir—CH3 bond, an iridium-vinyl complex, 12, is formed, which differs from the methane iridium-alkyl complex, 5, by having a much weaker agostic interaction between the methane and metal center. Scheme 4


145 Compared to 10, the Ir—vinyl bond in 12 is shorter by 0.124 A and the C—C double bond is longer by 0.021 A. These differences are indicative of a strong 7t-donating interaction between the vinyl ligand and the metal center of 12 as illustrated in Scheme 4. This interaction is favored over a ($-agostic interaction between metal center and vinyl ligand and over the agostic interaction between metal center and methane. Thus, the long Ir—methane distance in 12 is due to this electronic effect rather than to a steric effect.


The energy profiles at the B3LYP and CCSD//B3LYP levels are presented in Figure 4 and Table I (Reaction 2). The first step in the OA/RE reaction is the 7t-complexation of ethylene to the iridium center. Therc-complexis very stable; the ethylene association energy is -32.4 kcal/mol at the B3LYP level and -28.7 kcal/mol at the CCSD//B3LYP level (with the BSSE correction), respectively. This stabilization leads to a high barrier of 34 kcal/mol in the oxidative-addition step and a larger endothermicity of about 21 kcal/mol for the OA/RE product, 12. The high barriers and endothermicities make this reaction much more difficult than the OA/RE of methane, which is consistent with the fact that experimental observation of ethylene C — H bond activation and catalytic alkane dehydrogenation were not reported (6).

8 Figure 4. The B 3 L Y P energy profiles along the OA/RE pathways and from reactants, 2 and 4, to 12. C . Acetylene C — H Bond Activation. Recent experimental work shows that OA/RE and insertion reactions play important roles in the dimerization of terminal alkynes by late-transition metal catalysts (20). The cationic iridium-acetylene complex is similar to the iridium-ethylene complex in chemical behavior. As shown in Scheme 5, the acetylene 7i-complex (13) can undergo either oxidative-addition to give an iridiumacetylide complex or insert into an Ir—alkyl bond to give an iridium-vinyl complex. Thus, reactions of acetylene with the CpIr(PH3)(CH3)+ complex may result in several new products. In this part, we will consider and examine the possible steps in the acetylene C — H bond activation process.


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Scheme 5

The B3LYP optimized geometries for the ^-intermediate (13), oxidative-addition TS (14), oxidative-addition intermediate (15), reductive-elimination TS (16), and reductive-elimination product (17) along the OA/RE pathway are shown in Figure 5. The acetylenerc-complex,13, is similar to the ethylenerc-complex,8, both show longer Cp—Ir and Ir—-CH3 distances than those of iridium-alkane complex. The C—C triple bond is longer by 0.049 A than the C—C triple bond in free acetylene. These structural features show that there is a strong back-donating interaction between acetylene and the metal center. However, this interaction should be weaker than that of the iridium-ethylene 7C-complex as illustrated by smaller changes in C—C and Cp—Ir distances (vide infra).

Figure 5. The B3LYP optimized geometries for therc-intermediate(13), oxidative-addition TS (14), oxidative-addition intermediate (15), reductiveelimination TS (16), and reductive-elimination product (17) along the OA/RE pathway (only the average C—C and C—H distances are given for the Cp ring).


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As the acetylene rotates to put a C—H bond close to the metal center, the transition state, 14, forms in which the C—H bond is partly broken, while the Ir—acetylide and Ir—H bond are almost formed. Compared to the transition states for methane and ethylene C—H activation, 6 and 9, the transition state for acetylene C—H bond activation, 14, is slightly earlier with the shortest C—H and longest Ir—H distance. In the reductive-elimination TS, 16, the methyl—H distance is longer by 0.04 A than that of 6 and 11. After the hydrogen inserts into the Ir—methyl bond, a "non-agostic" iridium methane acetylide complex, 17, is formed, where the h—acetylide bond and the C—C distance are shorter than those of the oxidative-addition intermediate (15). Clearly, there is a strong 7C-donating interaction between the acetylide ligand and the metal center of 17 as there was in the Ir-vinyl complex, 12 (see Scheme 4). The energy profiles at the B3LYP and CCSD//B3LYP levels are presented in Figure 6 and Table I (Reaction 3).

Figure 6. The B3LYP energy profiles along the OA/RE pathway from reactants, 3 and 4, to 17. The acetylene OA/RE mechanism shares with its ethylene counterpart the feature of strong 7C-complexation between the hydrocarbon and the iridium center. The association energy of acetylene to the iridium complex, 4, -28.0 (B3LYP) and -20.1 (CCSD//B3LYP with the BSSE correction), is smaller by 4.0-8.6 kcal/mol than that of ethylene. Compared to ethylene, the acetylene reaction from the iridium 7C-complex, 13, to the iridium acetylide complex, 17, is less endothermic with a smaller barrier for the oxidative-addition step. Thus, the OA/RE reaction of acetylene with the iridium complex, although not facile, does take place more easily than that of the ethylene. In fact, there are several experimental reports about C—H bond activation of acetylene with late-transition metal complexes (20). Conclusions +

The alkane oxidative-addition from CpIr(PH3)(CH3)(alkane) to CpIr(PH3)(CH3)(H)(alkyl) is endothermic by about 0.8-4.4 kcal/mol with a low barrier of 10.0-11.5 kcal/mol, and reductive-elimination from CpIr(PH3)(CH3)(H)(alkyl) to a p-agostic +

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148 structure iridium-alkyl complex, CpIr(PH3)(alkyl)+ + CH4, is exothermic with a low barrier of 7.1-9.2 kcal/mol. Thus, the alkane C — H bond activation is a low temperature reaction process. Because of the strong association of ethylene and acetylene with CpIr(PH3)(CH3)+, the oxidative-addition/reductive-elimination (OA/RE) processes of ethylene and acetylene with CpIr(PH3)(CH3) are high energy processes, with barriers of 24-36 kcal/mol. Compared to ethylene, the OA/RE reaction of acetylene with CpIr(PH3)(CH3) complex is more favorable. On the other hand, the strong stabilizing interaction of ethylene with CpIr(PH3)(CH3) may lead to the following chemical equilibrium: the alkane dehydrogenation catalyzed by CpIr(PH3)(CH3) generates CpIr(PH3)(H)(rj C2H4) , lying toward the iridium-olefin product. Previous works have mentioned that the third-row transition-metal complexes undergo oxidative-addition, M + A — B -> M ( A ) ( B ) , more easily than their secondrow transition-metal congeners since late third-row transition metals have either d s ground states or d s low-lying excited states, while late second-row transition metals have d ground states with high-lying d s excited states (12,21). This preference emphasizes the importance of forming sd hybrid for the two new covalent bonds in the product. On the other hand, since the redox ability of transition-metals is reflected by their ionization potential (IP), the energy gap between the metal and ligand is directly proportional to the IP of the transition-metal. Thus, the oxidative-addition reaction, M + A — B —> M (A)(B), for M = Ir proceeds more easily than that for M = Rh because Ir(III) has a smaller ionization energy than Rh(III) (7b,22). Following the ionization potential of late transition-metals, one can predict that the ease of oxidative-addition is: Os(II) > Ru(II) > Pt(II) > Pd(II). +

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Acknowledgment. We thank the Robert A . Welch Foundation (Grant A-648) and the National Science Foundation (grants 94-23271 and 95-28196) for financial support. This research was conducted in part with use of the Cornell Theory Center, a resource for the Center for Theory and Simulation in Science and Engineering at Cornell University, which is funded in part by the National Science Foundation, New York State, and I B M Corporation. References (1) (a) Elschenbroich, Ch.; Salzer, A. Organometallics, VCH Publishers, New York, 1989. (b) Crabtree, R. H. The Organometallic Chemistry of the Transition Metals, John Wiley & Sons, New York, 1988. (b) Parshall, G. W. Homogeneous Catalysis, Wiley: New York, 1980 (2) (a) Koga, K.; Morokuma, K. Chem. Rev. 1991, 91, 823. (b) Musaev, D. G.; Morokuma, K. Advances in Chemical Physics, Volume XCV, Prigogine, I.; Edited by Rice, S. A. John Wiley & Sons, New York, 1996, 61. (c) Siegbahn, P. E. M.; Blomberg, M. R. A. Theoretical Aspects of Homogeneous Catalysts, Applications of Ab Initio Molecular Orbital Theory, Edited by Leeuwen, van P. W. N. M.; Lenthe, van J. H.; Morokuma, K. Kluwer Academic Publishers, Hingham, MA, 1995. (3) Reviews in Computational Chemistry, Vol. 1-7, Edited by Lipkowitz, K. B.; Boyd, D. B., New York, N.Y. : VCH, 1990-1996 (4) (a) Watson, P. L. J. Am. Chem. Soc. 1983, 105, 6491. (b) Watson, P. L.; Parshall, G. W. Acc. Chem. Res. 1985, 18, 51. (5) (a) Janowicz, A. H.; Bergman, R. G.J.Am. Chem. Soc. 1982, 104, 352.


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(b) Hoyano, J. K.; Graham, W. A. G. J. Am. Chem. Soc. 1982, 104, 3723. (c) Crabtree, R. H.; Mellea, M. F.; Mihelsie, J. M.; Quick, J. M.J.Am. Chem. Soc. 1982, 104, 107. (d) Jones, W. D.; Feher, F. J. J. Am. Chem. Soc. 1982, 104, 4240. (6) (a) Arndtsen, B. A.; Bergman, R. G. Science 1995, 270, 1970. (b) Burger, P.; Bergman, R. G.J.Am. Chem. Soc. 1993, 115, 10462. (c) Lohrenz, J. C. W.; Hacobsen, H. Angew. Chem. Int. Ed. Engl. 1996, 35, 1305. (7) (a) Strout, D.; Zarić, S.; Niu, S.-Q., Hall, M. B. J. Am. Chem. Soc. 1996, 118, 6068. (b) Su, M.-D.; Chu, S.-Y. J. Am. Chem. Soc. 1997, 119, 5373. (c) Hinderling,C.;Plattner, D. A.; Chen, P. Angew. Chem. Int. Ed. Engl. 1997, 36, 243. (d) Hinderling,C.;Feichtings, D.; Plattner, D. A.; Chen, P. J. Am. Chem. Soc. 1997, 119, 10793. (8) (a) Parr, R. G.; Yang, W.; Density-functional theory of atoms and molecules , Oxford Univ. Press: Oxford, 1989. (b) Becke, A. D. Phys. Rev. 1988, A38, 3098. (c) Becke, A. D. J. Chem. Phys. 1993, 98, 1372. (d) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (e) Lee,C.;Yang, W.; Parr, R. G. Phys. Rev. 1988, B37, 785. (9) Schlegel, H. B. Theor. Chim. Acta. 1984, 66, 33 (10) (a) Bartlett, R. J. Ann. Rev. Phys. Chem. 1981, 32, 359. (b) Scuseria, G. E.; Schaefer,IIIH. F. J. Chem. Phys. 1989, 90, 3700. (11) (a) Song, J.; Hall, M. B. J. Am. Chem. Soc. 1993, 115, 327. (b) Sulfab, Y.; Basolo, F.; Rheingold, A. L. Organometallics 1989, 8, 2139. (12) Jiménez-Cataño, R.; Niu, S.-Q.; Hall, M. B. Organometallics 1997, 16, 1962. (13) (a) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299. (b) Couty, M.; Hall, M. B. J. Comput. Chem. 1996, 17, 1359. (14) (a) Dunning, T. H., Jr.; Hay, P. J. in Modern Theoretical Chemistry , ed. H.F. Schaefer,III.Plenum: New York, 1976. (b) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284. (c) The BSSE correction for the reaction systems were performed using model systems: (H)Ir(PH )(H)(L) (L = CH , C H , and C H ). (15) Guest, M. F.; Kendrick, J.; van Lenthe, J. H.; Schoeffel, K.; Sherwood, P. GAMESS-UK; Daresbury Labratory: Warrington, WA4 4AD, U. K., 1994. (16) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T. A.; Petersson, G. A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski, V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. P.; Head-Gordon, M.; Gonzalez,C.;Pople, J. A. Gaussian 94 (Revision D.2); Gaussian, Inc., Pittsburgh PA, 1995. (17) (a) Bailey, N. A.; Jenkins, J. M.; Mason, R.; Shaw, B. L. J. Chem. Soc., Chem. Commun. 1965, 237. (b) LaPlaca, S. J.; Ibers, J. A. Inorg. Chem. 1965, 4, 778. (c) Cotton, F. A.; Day, V. W. J. Chem. Soc., Chem. Commun. 1974, 415. (d) Dwoodi, Z.; Green, M. L. H.; Mtetwa, V. S. B.; Prout, K.; Schultz, A. J.; Williams, J. M.; Koetzle, T. F.J.Chem. Soc., Dalton Trans. 1986, 802, 1629. (e) Dwoodi, Z.; Green, M. L. H.; Mtetwa, V. S. B.; Prout, K. J. Chem. Soc., Chem. Commun. 1982, 802, 1410. (f) Brookhart, M.; Green, M. L. H.; Wong, L.-L. Inorg. Chem. 1988, 30, 1. +

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(18) (a) The optimized geometry of CpIr(PH )(CH ) shows a similar structural feature to CpIr(PH )(H)(CH )(R)+ [R = CH (7), C H (14), and C H (26)] with a slipped Cpringand coordinating fashion. Niu, S.-Q.; Hall, M. B. unpublished work. (b) Zaric, S.; Hall, M. B. J. Phy.Chem. 1991,101, 4646. (19) (a) Niu, S.-Q.; Hall, M. B. J. Am. Chem. Soc. submitted. (b) Niu, S.-Q.; M. B. unpublished work. (c) Roubi, M. C&E News 1997, 75(49), 25. (d) Xu, W.-W.; Rosini, G. P.; Gupta, M.; Jensen,, C. M.; Kaska, W. C.; KroghJespersen, K.; Goldman, A. S. Chem. Commun. 1997, 2273. (20) (a) Crackness, R. B.; Orpen, A. G.; Spencer, J. L. J. Chem. Soc., Chem. Comm. 1984, 326. (b) Brookhart, M.; Schmidt, G. F.; Lincoln, D.; Rivers, D. S. Transition Meral Catalyzed Polymerizations, Quirk, R., Ed., Cambridge Univ. Press, 1988, (c) Schmidt, G. F.; Brookhart, M. J. Am. Chem. Soc. 1985, 107, 1443. (d) Trost, B. M.; Sorum, M. T.; Chan, C.; Harms, A. E.; Rühter, G. J. Am. Chem. Soc. 1997, 119, 698. (e) Slugovc, C.; Mereiter, K.; Zobetz, E.; Schmid, R.; Kirchner, K. Organometallics 1996, 15, 5275. (21) (a) Low, J. J.; Goddard, W. Α., III, J. Am. Chem. Soc. 1986, 108, 6115. (b) Low, J. J.; Goddard, W. Α., III, J. Am. Chem. Soc. 1984, 106, 8321. (c) Low, J. J.; Goddard, W. Α., III, J. Am. Chem. Soc. 1984, 106, 6928. (22) (a) Shriver, D. F.; Atkins, P. W.; Langford, C. H. Inorganic Chemistry, W. H. Freeman and Company, New York, 1990. (b) The IP values of transition-metal for high valence states can be obtained from the experimertal IP values for lower valence states by extrapolation. 3


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Transition State Modeling for Catalysis - American Chemical Society

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